1. Field of the invention
The present invention relates to an endoscope, and more specifically an optical system for endoscopes.
2. Description of the prior art
In the recent years, there have been increasingly used endoscopes so adapted as to observe object images on TV units while forming object images with solid-state image sensors such as CCD arranged in the distal ends, as well as endoscopes equipped with laser scalpels.
However, since the solid-state image sensors have high sensitivity to the infrared light, the sensors may form object images different from those observed by naked eyes, thereby causing misjudgment of affected parts. Further, since the YAG laser beam having a spectra component in the infrared region (approximately at 1061 nm) is used as the light source for the laser scalpels, the solid-state image sensors may be overflowed by the intense infrared light when the laser scalpels are operated, thereby whitening the entire screen surfaces (the same phenomenon as that caused by too bright light) and making object images invisible. Furthermore, since the YAG laser emits very intense light, the observer is placed in a very dangerous circumstance when the very intense light is reflected on an object and reaches to the observer's eyes through an observation optical system. Therefore, it is general to weaken the laser, before attaining to the solid-state image sensors and human eyes, by arranging infrared light cut filters at adequate positions in the objective optical systems. The infrared light cut filters are of a type which reduces the infrared light, before attaining to the solid-state image sensors and human eyes, by absorbing the infrared light or another type which reduces the infrared light, before attaining to the solid-state image sensors and human eyes, by reflecting the infrared light. Though the infrared light cut filters generally used have a function to reduce infrared light intensity down to some fractions on the order at the second to third position below decimal point, it is necessary to serially arrange two or more filters at a time when the infrared light intensity is high.
FIG. 1 shows a typical example of the conventional objective optical systems for endoscopes using infrared cut filters. In this drawing, the reference numeral 1 represents an image forming lens, the reference numeral 2 designates a stop arranged in the image forming lens 1, the reference numeral 3 denotes a YAG laser light absorption type of cut filter, the reference numeral 4 represents a YAG light reflection type of interference filter formed on the incidence surface of the YAG light absorption type of cut filter, the reference numeral 5 designates an infrared light absorption type of cut filter, the reference numeral 6 denotes an optical low-pass filter consisting of a quartz plate, etc. These optical members form an objective optical system A for endoscopes. The reference numeral 7 represents a solidstate image sensor having a protective glass 8 bonded to the sensitive surface thereof. The spectroscopic characteristics of the YAG laser light absorption type of cut filter 3, YAG laser light reflection type of interference filter 4 and infrared light absorption type of cut filter 5 are shown in FIG. 2.
Though the objective optical system A having the formation described above is capable of eliminating the infrared light almost completely, said objective optical system A comprises two infrared light absorbing filters 3 and 5 and inevitably has a long total length, thereby making it impossible to solve the essential problem of compact design of the distal end demanded for the endoscopes having small diameters.
In the next place, the objective optical system of the type which reduces the infrared light by reflecting it before attaining to the solid-state image sensor 7 poses a problem that transmittance is not lowered sufficiently due to multiple reflections between the filters when two filters are arranged parallelly and serially in the optical path. Speaking concretely, the light reflected by the second filter is reflected again by the first filter arranged before (on the side of incidence) the second filter, returns again to the second filter and the light partially transmits the second filter, thereby increasing transmission light intensity as compared with that in a case where a single filter is used. Especially, on an assumption that reflection is repeated infinitely between the two filters, total transmittance of the two filters cannot be lower than 1/2 of the transmittance of the filter having the lower transmittance. Accordingly, the objective optical system of this type cannot eliminate infrared light sufficiently and poses a problem that the infrared light hinders observation and constitutes danger during observation.
Though such a problem is not posed, needless to say, when the absorption type of filters shown in FIG. 1 are used, the absorption type of filters are lower in durability (affected more easily especially by humidity) than the interference film filters. When it is obliged to use the interference film filters or when an endoscope is to be used for medical purposes, it may therefore be convenient to design an objective lens system as an oblique view type. As a conventional objective optical system for endoscopes which deflects the viewing direction by utilizing the refraction, there is known an objective lens system comprising an objective optical system A consisting of a prism 9 arranged in the distal end of an endoscope and an image forming lens 1 and an image guide 10, as shown in FIG. 3A. Though this arrangement is often adopted when deflection angle is within 20.degree., the entrance surface of the prism 9 inclines to the optical axis 0 and its exit surface is perpendicular to the optical axis. Accordingly, the light entering to the center of visual field or the center of the entrance end face of the image guide 10, that is, the light progressing along the optical axis 0 is deflected by a desired angle .phi. only with the refraction at the entrance surface 9a of the prism 9 as shown in FIG. 3B. In case of an endoscope using an objective lens system performing deflection by utilizing the refraction by the surface 9a only located on the side nearest the object as in the case of this conventional example, there are posed problems that the distal end of the endoscope must be inclined and that the inclination angle is too large relative to the effect obtained by the deflection. Further, there are posed problems that, when the endoscope is used in water or another medium, the deflection is reduced remarkably as compared with that in air, thereby making it impossible to obtain sufficient deflection of viewing direction, and that the deflecting action produces distortion, astigmatism and chromatic aberration too grave for the deflection effect since the deflection is performed by utilizing refraction on a single surface.